Thin films can be produced from polymers of high molecular weights, in particular ultrahigh molecular weights, by the dissolution process or swelling process, in such a way that the polymers are dissolved or swollen in a solvent, and the solution or swollen material is shaped to give a film which is then cooled to a temperature below the gelling temperature of the polymer, whereupon the film is stretched simultaneously or sequentially in the longitudinal and transverse directions after at least partial removal of the solvent.
Especially when ultrahigh stretching ratios (stretching ratios of more than five in each direction in the case of simultaneous stretching) are maintained, the problem arises that the gelled films obtained after cooling are more extensively stretched in the areas, at which they are gripped by the clamps of the stretching device, than in the areas not gripped by the clamps, which entails, with increasing stretching ratios, the formation of increasingly thinning lips which can tear when higher stretching ratios are reached. Furthermore, the film segments adjoining these lips have a greater anisotropic orientation than the segments which adjoin the lateral film segments not gripped by the clamps. Accordingly, the properties are anisotropic (unequal) at the film edges, so that the edge segments of films, especially those stretched at ultrahigh stretching ratios, must be cut off to a considerable width, which causes waste quantities of 20 to 50%.
Moreover, there is a demand for the production of stretched thin films which, for decorative or handling reasons or for fixing purposes, must have stiffened segments. This demand remains essentially unfullfilled.